1. Field of the Invention
The present invention relates generally to an optical communication system and an optical amplifier suitable for long-haul and large-capacity transmission, and more particularly to an optical communication system applicable to WDM (wavelength-division multiplexing) and an optical amplifier suitable for WDM.
2. Description of the Related Art
In recent years, research and development on application of an optical amplifier to an optical communication system have been intensively pursued. For example, importance of a booster amplifier, optical repeater, and preamplifier having an EDFA (erbium doped fiber amplifier) has become apparent.
Conventionally known is an optical amplifier comprising an optical amplifying medium for amplifying signal light and means for pumping the optical amplifying medium so that the optical amplifying medium has an amplification band including the wavelength of the signal light. In the case that the optical amplifying medium is an EDF (erbium doped fiber) having a first end and a second end, the pumping means includes a pump light source for outputting pump light having a proper wavelength, and means for supplying the pump light into the doped fiber from at least one of the first end and the second end. In the case that the optical amplifying medium is provided by a semiconductor chip, the pumping means includes means for injecting a current into the chip.
To greatly increase a transmission capacity, a WDM system (wavelength-division multiplexing system) has been proposed. The WDM system includes a first terminal station for outputting WDM signal light (wavelength-division multiplexed signal light) obtained by wavelength-division multiplexing a plurality of optical signals having different wavelengths, an optical transmission line for transmitting the WDM signal light output from the first terminal station, and a second terminal station for receiving the WDM signal light transmitted through the optical transmission line. To increase a transmission distance in the WDM system, one or more optical repeaters each having an optical amplifier are provided in the optical transmission line.
In applying the optical amplifier to the WDM system, gain tilt occurring in the optical amplifier must be considered. The gain tilt is based on the wavelength dependence of gain. In an EDFA, for example, the gain tilt changes with a change in total input power because of characteristics of homogenous broadening of an EDF. Accordingly, in operating the WDM system or the optical repeater, it is desirable to grasp the gain tilt of the optical amplifier and maintain a constant gain tilt.
In the optical amplifier or the optical repeater, a feedback loop for ALC (automatic level control) is usually adopted, so as to maintain the output level constant. In applying the optical amplifier adopting ALC to the WDM system, a target level in ALC for maintaining output power per channel constant changes with a change in the number of channels of WDM signal light. Accordingly, the conventional optical communication system cannot easily respond to a change in the number of channels.